This application claims benefit of Japanese Patent Application No. 2002-316928 filed on Oct. 31, 2002, the contents of which are incorporated by the reference.
The present invention relates to phase modulators and, more particularly, to phase modulators for phase modulating digital signals used in radio communication systems.
Phase modulators used in radio communication and like technical fields were constructed as analog circuits. In recent advancement of digital circuits, however, a circuit up to a part for modulating a carrier as carrier wave tends to be provided in a digital circuit (see a first prior art: GRAYCHIP APPLICATION NOTES, BUILDING A QAM MODULATING USING A GC2011 DIGITAL FILTER CHIP, Oct. 6, 1994, Joseph H, Gray, pp. 1-4).
FIG. 10 is a block diagram showing a digital phase modulator shown in this prior art. An input signal, which is a digital signal in the form of a bit train, is converted in an IQ converter 21 to I and Q signals, which are inputted to level judgment units 22 and 23, respectively. The outputs of the level judgment units 22 and 23 are inputted to FIR filters 24 and 25 as digital filters, respectively, for frequency band restriction.
A carrier generator 26 is provided as a carrier source for generating orthogonal carriers (sine and cosine carriers), i.e., digital sine and cosine signals, which are 90 degrees out of phase from each other. The carrier generator outputs are fed to multipliers 27 and 28, respectively to be multiplied by the outputs of the filters 24 and 25. The multiplier outputs are combined in an adder 29. The adder output is converted in a D/A converter to an analog signal, which is provided as IF signal.
The phase modulator shown in FIG. 10 is a well-known QPSK modulator, and is thus not described in detail. As shown above, by preparing the digital sine/cosine generator 26, the circuits for generating a multiple-value digital signal, multiplying (i.e., phase modulating) the signal in the digital modulators 27 and 28 and adding (i.e., combining) the product results, can be constructed as digital circuits.
Also, in a second prior art (U.S. Pat. No. 4,680,556) a digital phase modulator is disclosed, in which preliminarily calculated values obtained in correspondence to I and Q signal train combinations are written in a ROM (read-only memory) and, when required, memory data in an appropriate address is read out and D/A converted.
In the digital phase modulator shown in FIG. 10, digital FIR filters are used as the filters 24 and 25. This is done so because of the fact that the digital signal processing has to be carried out by calculating the intended pulse signal by taking the response of several preceding and succeeding pulses to considerations instead of calculating modulation output concerning a single signal (or pulse). The FIR filters, as is well known in the art, each have a multiple-stage multiplier, thus leading to large circuit scale and high power consumption. Furthermore, it takes a time for obtaining the filter outputs.
While in the digital phase modulator shown in the second prior art preliminarily calculated values are written in the ROM and, when required, memory data in an appropriate address is outputted and fed to the D/A converter, as shown in FIG. 3 in the second prior art, for determining the waveform of a digital pulse a waveform process which takes the effects of a plurality of pulses including preceding and succeeding pulses into considerations is necessary.
In such case, that is, in the case when it is intended to have a change on a time axis to correspond to a single pulse instead of carrying out the waveform process taking pluralities of preceding and succeeding pulses into considerations for the determination of the waveform of a single pulse, it may be thought to use a raised cosine pulse as disclosed in a third prior art (McGRAW-HILL BOOK COMPANY, INTER-UNIVERSITY ELECTRONICS SERIES, VOL. 2 DATA TRANSMISSION, 1965, Williams R, Bennett, et al. pp. 50-53). In this connection, an ideal rectangular pulse as shown in FIG. 11 has a great frequency spectrum width (or frequency band) and therefore cannot be processed in an actual circuit. By using a raised cosine pulse as shown in FIG. 12, the frequency band is more restricted, that is, it is made so narrow that the waveform of a pulse can be determined without consideration of the effects of preceding and succeeding pulses.
However, the waveform of this raised cosine pulse, however, has a drawback that the aperture of the so-called eye is maximum only at a momentary instant. The fact that the eye aperture is maximum only at a momentary instant is very disadvantageous for the determination of the waveform of a pulse having jitter, and gives rise to a reliability problem.
Furthermore, in a four-phase modulator, in the case where the phase-modulated wave vector is changed between the first and third quadrants and the second and fourth quadrants in the IQ phase plane, it passes through the origin on the IQ phase plane as shown in FIG. 3, and therefore, the phase-modulated signal envelope momentarily becomes zero. Such a signal has a disadvantage that it is distorted when being passed through a high output high frequency amplifier due to non-linear characteristics (saturation characteristic) of the amplifier.